I have a doubt about the load balancing with a asa5510 with security plus (asa5510-sec-bun-k9).
I have a specific question:
how many logical or phisycal devices (asa5510) are necesary to implement load balancing?.
ASA does not do load balancing.
You can configure ASA with multiple context, and each context can be active on either ASA, however, it does not perform traffic load balancing.
If you have 3 contexts, ie: Context A, Context B and Context C (all 3 are virtually separate firewalls):
Context A - can be active on ASA-1 (and standby on ASA-2)
Context B - can be active on ASA-2 (and standby on ASA-1)
Context C - can be active on ASA-2 (and standby on ASA-1)
However, traffic from Context A can't be load balanced between ASA-1 and ASA-2.
Hope that helps.